Allow number of commit log buckets to be increased

Also increases the number of commit log buckets on alpha to 397 and correspondingly
reduces the frequency of commit log diff exporting to once every 3 minutes.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=191440586

java/google/registry/backup/ExportCommitLogDiffAction.java

@@ -14,6 +14,7 @@
 
 package google.registry.backup;
 
+import static com.google.common.base.MoreObjects.firstNonNull;
 import static com.google.common.base.Preconditions.checkArgument;
 import static com.google.common.base.Verify.verifyNotNull;
 import static com.google.common.collect.ImmutableList.toImmutableList;
@@ -175,10 +176,13 @@ public final class ExportCommitLogDiffAction implements Runnable {
       @Nullable CommitLogCheckpoint lowerCheckpoint,
       CommitLogCheckpoint upperCheckpoint,
       int bucketNum) {
-    // If no lower checkpoint exists, use START_OF_TIME as the effective exclusive lower bound.
-    DateTime lowerCheckpointBucketTime = lowerCheckpoint == null
-        ? START_OF_TIME
-        : lowerCheckpoint.getBucketTimestamps().get(bucketNum);
+    // If no lower checkpoint exists, or if it exists but had no timestamp for this bucket number
+    // (because the bucket count was increased between these checkpoints), then use START_OF_TIME
+    // as the effective exclusive lower bound.
+    DateTime lowerCheckpointBucketTime =
+        firstNonNull(
+            (lowerCheckpoint == null) ? null : lowerCheckpoint.getBucketTimestamps().get(bucketNum),
+            START_OF_TIME);
     // Since START_OF_TIME=0 is not a valid id in a key, add 1 to both bounds. Then instead of
     // loading lowerBound < x <= upperBound, we can load lowerBound <= x < upperBound.
     DateTime lowerBound = lowerCheckpointBucketTime.plusMillis(1);